During operation of a gas turbine engine, the fan thereof draws the working medium gases, more particularly air, into the engine. The fan raises the pressure of the air drawn along the secondary flow path, thus producing useful thrust. The air drawn along the primary flow path into the compressor section is compressed. The compressed air is channeled to the combustor section, where fuel is added to the compressed air, and the air-fuel mixture is burned. The products of combustion are discharged to the turbine section. The turbine section extracts work from these products to power the fan and compressor. Any energy from the products of combustion not needed to drive the fan and compressor contributes to useful thrust.
One critical concern in the fabrication of gas turbine engines is the overall weight of the engine. Excessive weight in the components of the gas turbine engine limits the useful load the engine can power and reduces the flight range capability of the aircraft. Thus, it is a goal of the gas turbine engine industry to minimize the overall weight of the engine without sacrificing the performance or durability thereof.
It is this effort to minimize the overall weight of the gas turbine engine that has led the industry to the use of hollow fan blades. Each hollow fan blade typically includes two outer skins joined at both the leading and trailing edges and defining a hollow interior cavity therebetween. The hollow interior cavity has a plurality of internal spanwise and chordwise stiffening ribs disposed therein which further divide the interior into a plurality of hollow cavities.
Federal Aviation Administration (FAA) certification requirements for a bladed turbofan engine specify that the engine demonstrate the ability to survive failure of a single fan blade at a maximum permissible rotational speed, such failure being hereinafter referred to as the "blade loss condition." The certification tests require containment of all blade fragments and the safe shutdown of the engine. The ideal design criterion is to limit the damage caused by the single released blade, such that the released blade should not cause any other blade to fracture and be released. Impact loading on the containment casing and unbalanced loads transmitted to the engine structure are then at a minimum. If fan imbalance becomes too great, loss of the entire fan, engine or engine support structure can result.
The certification test method includes intentionally releasing a fan blade from the supporting hub by using both mechanical and explosive means. The released blade travels radially outward in the flow path with velocities of several hundred feet per second. Past experience has shown that when prior art hollow fan blades fracture at the outer portion of the dovetail attachment, the released blade will impact the leading edge of the adjacent blade following the released blade relative to the direction of rotation, hereinafter referred to as "following blade". The released blade may also impact the following blade at the trailing edge. As a result of the blade impacts, the following blade may fracture. These fractures will initiate at or in close proximity to the points of impact. The fractures may lead to the loss of a major portion of the following blade.
In addition, the loss of a major portion of the following blade leads to additional imbalance in the engine, which requires strengthening of the engine structure, including the containment system, the engine rotor and casing, the rotor bearing structures, the engine mounts and all the engine supporting structure including the wing and fuselage.
There are several possible solutions to the problem of severed fan blades due to the impact of a released blade with adjacent blades. One solution would be the addition of stronger platforms between the blades. The platforms can be mounted on the hub between the blades and as such, will not be an integral part of the blades. These platforms would offer resistance to the trajectory of the released blade as the released blade would be prevented from traveling through the space occupied by the added platform. Thus, the primary impact with the following blade would be absorbed by the stronger platform. As a result, the trajectory of the released blade would be altered such that the released blade would secondarily impact the following blade further outboard of the span of the following blade. The platform could delay the impact of the released blade upon the following blade and any possible fracture would also occur radially further outboard of the blade span. The resultant damage could thus be minimized as only a smaller portion of the following blade would be susceptible to loss. However, the addition of platforms between the hollow fan blades would have a significant impact on blade weight, fan performance and engine weight and thus be undesirable. Another possible solution would be to increase the thickness of the spanwise ribs and add chordwise ribs locally in the impact regions of the blade. These rib structures would make the blade more rigid against chordwise deformation. However, this structural reinforcement is less likely to prevent impact induced crack initiation, and may only have limited success in the prevention of subsequent crack propagation.